The possibility of detecting concealed people through their heartbeats has been considered. GeoVox Security, Inc. of Houston, Tex. sells an Avion heartbeat detector for security applications. Such detectors operate on the principle that a beating heart creates mechanical shock pulses as it pumps blood through a body. The shock pulses produce vibrations that propagate through the body and through objects in contact with the body.
The vibrations have a very small amplitude—a fraction of the width of a human hair. Nonetheless, sensors exist that can detect such small vibrations. For example, geophones are used in oil exploration. Geophones are sensitive enough to detect vibrations that emanate from a mechanical device and travel long distances through the earth.
A difficulty in using such small amplitude signals in security applications is that there are many other sources of similar signals with a similar or greater magnitude. For purposes of detecting a signal from a beating heart, these signals are noise. A security system is likely to mistake the noise for a signal representing a beating heart, creating a “false alarm.”
A false alarm is undesirable in a security system because of the cost of investigating each alarm. For example, in the case of a system checking cargo containers for stowaway passengers, an alarm generated for a container triggers a physical inspection of that container. Physically inspecting the container ties up security personnel and delays shipping operations. Where the inspection is undertaken in response to a false alarm, these costs are wasted. If a security system has a high false alarm rate, its output may be so unreliable that it is ignored or the cost of investigating false alarms may be so great that the system is not used at all.
Security systems are designed so as not to respond to noise and therefore lower their false alarm rates. However, many methods that a system could use to reject noise reduce the sensitivity of the system to a signal the system needs to detect. Reducing the sensitivity to the signal to be detected is also undesirable because it reduces the chances that the desired signal will be missed, creating a false positive. False positives are particularly undesirable in a security system because a threat might be passed undetected.
Accordingly, it is desirable for a security system to have a low false alarm rate while simultaneously providing a low rate of false positives. It would be highly desirable to provide an improved systems for detecting people and other animals from their heartbeats with a low false alarm rate while simultaneously providing a low rate of false positives.